A silver halide emulsion used in a silver halide photographic material is usually subjected to chemical sensitization using various chemical substances so as to obtain desired sensitivity and gradation. Known representative examples of the chemical sensitization are sulfur sensitization, selenium sensitization, tellurium sensitization, sensitization with a noble metal such as gold, reduction sensitization and various sensitization comprising a combination of these.
Recently, demands for a silver halide photographic material to provide high sensitivity, superior graininess and high sharpness and further for rapid processing involving expedited development procedure have been intensified and various improvements have been made on the above-described sensitization methods.
Among sensitization methods, sulfur sensitization, selenium sensitization and tellurium sensitization are generically called as chalcogen sensitization and in particular, sulfur sensitization is most fundamental sensitization and has hitherto been an object of a huge elaboration and improvement.
On the other hand, selenium sensitization is known to have greater sensitization effects than the above-described sulfur sensitization in a common use in the art and described in U.S. Pat. Nos. 1,574,944, 1,602,592, 1,623,499, 3,297,446, 3,297,447, 3,320,069, 3,408,196, 3,408,197, 3,442,653, 3,420,670 and 3,591,385, French Patents 2,693,038 and 2,093,209, JP-B-52-34491 (the term "JP-B" as used herein means an "examined Japanese patent publication"), JP-B-52-34492, JP-B-53-295, JP-B-57-22090, JP-A-59-180536 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), JP-A-59-185330, JP-A-59-181337, JP-A-59-187338, JP-A-59-192241, JP-A-60-150046, JP-A-60-151637, JP-A-61-256738, JP-A-3-4221, JP-A-3-148648, JP-A-3-111838, JP-A-3-116132, JP-A-3-237450, JP-A-4-16838, JP-A-4-25832, JP-A-4-140738, JP-A-4-140739, European Patent 506009A1, British Patents 255,846 and 861,984, and H. E. Spencer et al., Journal of Photographic Science, Vol. 31, pp. 158-169 (1986).
The tellurium sensitization has been remarkably progressed in recent years and found to have many advantages such as great sensitization effect as compared with sulfur sensitization, fast development, good reciprocity law characteristics, high color sensitization sensitivity and good pressure effect as described in JP-A-4-204640, JP-A-4-271341, JP-A-4-333043, JP-A-5-11381, JP-A-5-11387, JP-A-5-11388, JP-A-5-45769, JP-A-5-45770, JP-A-5-303157, JP-A-6-27573, JP-A-6-208184 and JP-A-6-208186.
However, although the selenium sensitization has greater sensitization effect than the sulfur sensitization commonly used in the art, it suffers from outstanding occurrence of fog and is also prone to cause softening. Further, although the tellurium sensitization has the above-described predominant property, when the tellurium sensitization is applied to a practical emulsion containing various additives or a gold sensitizer is used in combination to achieve higher sensitivity, unintended fog is generated. Accordingly, techniques for reducing fogging without impairing sensitivity and storability have been demanded.